Method for precisely-positioned production of a cavity, especially a bone cavity and instrument therefor

ABSTRACT

The invention relates to a method for precisely-positioned production of a cavity, especially a bone cavity, at a preparation point by means of a hand instrument, comprising the following steps: calculation of position-dependent surface characteristics from a three-dimensional data set of the surface of the preparation point at a desired position of an implant which is to be inserted in said cavity, wherein the area in which the cavity is to be created is represented in the form of a three-dimensional data set of volume data; detection of at least one partial cut-out of the preparation point comprising an actual visible surface feature by means of a camera which is arranged on the hand instrument at a pre-defined distance to a processing tool and display as a video image; insertion of a calculated surface characteristic for the desired position of the hand instrument, wherein the inserted surface characteristic can be altered, especially made congruent with respect to its position in relation to the actual visible surface characteristic by modifying the position and the inclination of the hand instrument.

BACKGROUND OF THE INVENTION

The invention relates to a method of creating a cavity, in particular abone cavity, in correct position, and to an instrument for carrying outsaid method.

Dental implantology involves the incorporation of exogenous fittings, orso-called implants, into the bone, in order to provide asubstance-protecting prosthetic replacement of missing teeth and toprovide edentate jaws with a fixed dental restoration.

In order to achieve long-lasting, successful treatment, various dentaland anatomical constraints must be observed when such fittings areimplanted. The transference of masticatory forces to the jaw-bone mustbe accomplished in such a way that the approved rules of biomechanicsand dental prosthetics are observed. Body cavities may not be opened andnerve tracts may not be damaged. Finally, it is necessary to ascertainwhether a quantitatively and qualitatively adequate supply of bonysubstance is present.

These constraints require making a thorough diagnosis prior to theintervention and planning a suitable strategy for carrying out theintervention. The ultimate goal is to achieve a good implementation ofthe strategy during the operation.

PRIOR ART

As a rule, the implantologist can use the currently available technologyaids such as computer-assisted tomography (CAT), orthopantomography(OPG), and measurements of bone density and bone thickness to make anadequate diagnosis and use it to devise a good implantation strategy.

There are, however, major shortcomings as regards putting this strategyinto practice.

In recent years attempts have been made to bridge this gap.Neurosurgical and orthopedic procedures have been used in doing so.Markings have been placed on the patient and the operating instrumentsfor observance with the aid of twin cameras. The relative position of,say, a drill can be determined from the known geometry of the operatinginstruments and the markings. Using this principle it is possible toimplement the implantion strategy effectively. With such procedures, ithas been possible to achieve precision levels of 1 mm, or in otherwords, deviations as great as 1 mm might occur between the planned andthe attained position.

The disadvantage of such equipment is its comparatively high price. Suchequipment is thus only economically feasible for the insertion of alarge number of implants.

It is thus an object of the invention to provide a more cost-effectivesolution to the problem described above, in order to make the insertionof only a few implants economically feasible.

DESCRIPTION OF THE INVENTION

The invention described herein is based on the idea that suitablesurface features, such as the horizon line of a topographic surface can,in encoded form, also provide information on the position of theobserver.

On this basis, a method of creating a cavity in a desired position at apreparation site is proposed, wherein, using a hand instrument,position-dependent surface features are computed from a threedimensional data set referring to the surface of the preparation site togive a target position for insertion of an implant into a cavity, thearea in which the cavity is to be created and the implant subsequentlyinserted being displayed as a three dimensional spatial data set.

A camera mounted on a hand instrument at a specified distance from amachining tool records and displays, as a video image, a section of thepreparation site showing at least one actually visibleposition-dependent surface feature. The computed position-dependentsurface feature for the target position of the hand instrument will besuperimposed on the video image, and by changing the position and angleof the hand instrument, the position of the superimposedposition-dependent surface feature can be changed relatively to theactually visible position-dependent surface feature and, in particular,be brought into coincidence therewith.

By the term “camera” we mean all measuring equipment, imaging equipment,or image digitizing equipment capable of recording a section of thepreparation site such that it can be displayed as a video image.

In this way it is possible to show the user whether his hand instrumentis oriented correctly to create the cavity as planned, so that theimplant can be inserted correctly at the target site.

The relative position of the X-ray image and the 3D data set is knownfrom special markings, so-called X-ray-opaque markers, which are visiblein both the spatial data of the X-ray image and in the 3D data setrelating to the surface. The insertion site for the implant isdetermined by the implantologist with the aid of the X-ray image. Inthis manner the position of the insertion site relative to the 3D dataset will also be known, and from this position the 3D data set is usedto compute the position-dependent surface feature which would resultwhen the drill is placed in the planned position and is angled tocoincide with the planned drill axis.

Any suitable, visually displayable feature of a tooth or the preparationarea that can be computed from the surface data and that enables adefinition of a position may be used as a surface feature, aparticularly suitable feature being a horizon line.

During machining, the computed position-dependent surface feature forthe respective position of the hand instrument is always advantageouslysuperimposed on the video image, wherein the position of the hand-heldinstrument corresponds to a position of the implant inside the cavity.This ensures that adjustments will still be possible even though thehand instrument would have been initially positioned correctly.

Advantageously, monitoring should be possible when the end position isreached, by showing a position-dependent surface feature for an endposition of the machining tool in the cavity to be created.

The invention also relates to a hand instrument for creating ormachining cavities, in particular bone cavities, which includes amachining tool. The hand instrument is equipped with a camera mounted ata known distance from the tip of the machining tool.

With such a hand instrument it is possible to scan the preparation siteand compare it with a three dimensional data set of spatial data orsurface data.

The camera advantageously has a depth of focus of from 5 to 30 mm andscans a panorama view. This permits orientation to the side showing themost pronounced orienting features of the 3D data set.

According to an additional embodiment, the video camera is built intothat end of the instrument which is near to the machining tool. Thispermits exact registration of the actual horizon line of the referenceimage at the preparation site.

Finally, light sources can be provided to illuminate that part of thesurface which is relevant for registering and displaying the horizonline, or to illuminate other distinctive features of the surface.

Furthermore, the hand instrument can be linked to a display for theimage recorded by the camera, which display may also show data, providedby an evaluating unit, in the form of a horizon line or some otherdistinctive surface features.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of the invention is illustrated below with reference to thedrawings, in which

FIG. 1 shows a horizon line in a first view,

FIG. 2 shows a horizon line in a second view,

FIG. 3 shows a tooth and a target horizon line coinciding therewith,

FIG. 4 illustrates correct positioning of the machining tool of the handinstrument,

FIG. 5 shows a tooth and a non-coincident target horizon line,

FIG. 6 illustrates incorrect positioning of the machining tool of thehand instrument,

FIG. 7 illustrates correct positioning of the machining tool of the handinstrument during machining,

FIGS. 8-11 illustrate how the horizon line changes during machining upto an end position,

FIG. 12 shows diagrammatically a setup for carrying out the method.

The concept of the horizon line as an example of a position-dependentsurface feature is explained with the aid of FIGS. 1 and 2. A horizonline is characterized by the following imaging rule. An observer atpoint A constructs, say, a circular projection screen P around himself,which is covered with a transparent film and has a known radius r. PointB for each angle element alpha of the entire panorama is marked onscreen P, and horizon C appears to lie on point B for the observerlocated at center point A. The image thus generated is designated ashorizon line H. The distinguishing feature of horizon line H is that thelocation of the observer is also implicitly coded in its shape.Conversely, when an adequately defined, unique horizon line is known, itis possible to relocate the place from which said horizon line wasobserved.

It depends on the displayed objects themselves whether there is a clearcorrelation between the horizon line and the displayed objects. The morecomplex the objects are, the clearer the correlation will be. Thefollowing example illustrates this situation. From the photograph of amountain range, one can determine the location from which thisphotograph was taken, if one also knows the route that was taken alongthe mountain range.

The spatial relationship between the measurement data recorded bysurface scanning and the X-ray-generated spatial data is made known bythe use of X-ray-opaque markers. The implant insertion site isdetermined with the aid of an X-ray image (OPG, CAT). The surface datacoordinates thus obtained are used to monitor correct positioning of thetool while creating the cavity.

In doing so, the horizon lines of adjacent teeth 1, 2 or other suitableposition-dependent surface features are taken into account.

The expected horizon lines along the direction of implant insertion arecomputed with the aid of the 3D data set. Thus, all points A lie on anaxis E parallel to, but in particular coincident with, the insertionaxis.

FIG. 3 shows a diagram of a tooth 1, as it appears on a display whenrecorded through the optics of the machining tool. The thick linedesignates the horizon line H, which would be visible at the desiredposition of implant axis as defined in the strategy. In this case thetrue horizon and the horizon line are coincident. The present positionof the drill is thus correct.

FIG. 4 shows a machining instrument 3 having a tool 4, which is locatedat the predetermined position. At its end 5 near machining tool 4, handinstrument 3 possesses optical means accommodated in a small cylindricalattachment 6 and designed to record the surrounding area, for example abuilt-in intra-oral video camera. The distance between the tip 7 of thetool and the position of said optical means is known. Attachment 6 islocated above tool 4 and accommodates the optical means and also anoptical fiber bundle to illuminate the teeth.

The optics should have a field depth ranging from 5 to 30 mm. It mustnot be telecentric, that is, objects at different distances must bedisplayed as different in size. It may comprise 360 degree panoramaoptics, as indicated in FIG. 6. It is sufficient, however, to use animage field large enough to display one tooth. Thus the camera will thenregister tooth 1, 2 only from one direction.

In this way the adjacent teeth 1 and 2 are displayed in the model pointof the implant drill hole 8 to be formed. It is important for thepresent procedure that the protuberances of the teeth stand out well, ina manner similar to the crests of mountains. Other distinct surfacefeatures may be provided, for example, by fillings. Here too,essentially all visible, position-dependent surface features areregistered.

No special requirements apply as regards the optical distortion of thesystem. Essentially, a monochrome image is sufficient.

FIG. 5 shows a diagram of a display, in which the true horizon W and thehorizon line H are not coincident. The present position of the drill isnot correct, see FIG. 6. The drill is not yet in the predeterminedposition.

When the drill is in the correct position, as shown in FIG. 7, theoperator begins to drill into the bone. The system now displays thathorizon line to which the user may drill. FIG. 8 shows the properlypositioned tool 4 of hand instrument 3. FIG. 9 shows the associatedhorizon line. The matching horizon line H in the background shows thatthe model point for drilling has been correctly found. A second,non-matching horizon line H′ in the foreground becomes visible followingcommencement of drilling. When the drill reaches its computed endposition according to FIG. 10, this horizon line H′ will then match thedisplayed image of the tooth, see FIG. 11. The scene filmed by thecamera changes during the drilling operation so that the horizon line H′will now be coincident with the visible horizon.

The method is carried out as follows: the surface of tooth or jaw ismarked with X-ray-opaque markings, which enable correlation of 3Dspatial measurement data obtained from X-ray images with 3D dataobtained by scanning the surface. The X-ray-opaque markers are visiblein the X-ray image as well as in the surface image. They make itpossible to define the location of the two images relative to eachother.

An implantation strategy is worked out with the aid of the X-ray images.This includes defining the drilling depth and the angle of the drilledchannel.

The horizon line associated with all positions of the machining toolalong the drilled channel is computed from the 3D surface-scanning data.

The instrument for creating the implantation hole has an intra-oralvideo camera positioned at a known distance from the tip of the drillinginstrument. The computed horizon line for the target position is scannedinto the video image produced by this camera, with all opticaldistortions accounted for. The superimposed horizon line can be broughtinto coincidence with the actually visible horizon by changing theposition and angle of the drilling instrument.

During creation of the implantation hole, the horizon lines thusgenerated are viewed continuously to ensure that drilling continues inthe correct direction.

A setup is illustrated diagrammatically in FIG. 12. Unit 11 comprises adisplay 12, on which images recorded by hand instrument 3 can bedisplayed. At the same time, an evaluating unit 13 controls the drilldriver and computes the surface feature, in this case the horizon line.The instrument can be con-trolled such that the machining tool driverwill be shut off when a deviation from the specified position isdetected and released for a restart only when the correct position hasbeen regained.

LIST OF REFERENCE NUMERALS OF CHARACTERS

-   1 tooth-   2 tooth-   3 hand instrument-   4 tool-   5 end of the hand instrument-   6 attachment-   7 tip of tool 4-   8 drill-hole-   9-   10-   11 unit-   12 display-   13 evaluating unit-   A position/center of the viewer-   B point-   C horizon-   E axis-   H horizon line-   H′ horizon line-   P projection screen-   R radius-   W

1. A method of moving a hand instrument which includes a machining tooland a camera located at a specific distance from the machining tool toprovide a cavity in a bone for a dental implant at a preparation site,comprising the following steps: computing position-dependent surfacefeatures of a three-dimensional data set relating to a surface of anarea at the preparation site relative to a desired position of theimplant, the area in which the cavity is to be created being adjacent atooth defining a horizon line and present in the form of athree-dimensional set of volume data; detecting at least one section ofthe preparation site which exhibits a visible real surface feature bymeans of the camera on the hand instrument and a display providing avideo image; and superimposing a computed surface feature for a targetposition of said hand instrument such that altering the position andangle of said hand instrument causes a change in the position of saidsuperimposed surface feature relative to the visible real surfacefeature, and moving the hand instrument relative to the preparation siteuntil the computed horizon line coincides with the horizon line of theadjacent tooth.
 2. A method as defined in claim 1, wherein during amachining operation always those surface features which are computed forthe current position of said hand instrument are superimposed over saidvideo image, the current position of said hand instrument correspondingto a position of the implant within said cavity.
 3. A method as definedin claim 1, wherein the machining tool includes a tip whose distancefrom the camera is known, wherein the hand instrument transmits an imageproduced in the camera to said display, and wherein the computed horizonline is computed in an evaluating unit to enable the hand instrument tobe controlled when creating or excavating a cavity in a bone to an endposition wherein the computed horizon lines of an adjacent toothcoincides with the visible camera-generated horizon line of the tooth.